Ever wonder what keeps your heart beating? That would be a bundle of nerves called the sinoatrial node. It produces the electrical signal that makes your heart contract and relax like an angry milkmaid's fist.

About 60 to 70 times a minute when you're resting, this node jolts the rest of the heart into action, sending blood through the heart's chambers and re-oxygenating it before sending it on its way through the rest of the body.